Probá así:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>ejemplo</title>
<script>
function validar(e){
obj=e.srcElement || e.target;
tecla_codigo = (document.all) ? e.keyCode : e.which;
if(tecla_codigo==8)return true;
patron =/[d.]/;
tecla_valor = String.fromCharCode(tecla_codigo);
control=(tecla_codigo==46 && (/[.]/).test(obj.value))?false:true
return patron.test(tecla_valor) && control;
}
</script>
</head>
<body>
<form id="form1" name="form1" method="post" action="">
<textarea name="textarea" onkeypress="return validar(event)"></textarea>
<textarea name="textarea2" onkeypress="return validar(event)"></textarea>
</form>
</body>
</html>